1. Field of the Invention
The present invention relates to a transmission system using a multimode fiber transmission line.
2. Description of the Related Art
When a multimode fiber (MMF) transmission is performed, it is desired that MMF dedicated equipment is used. However, since most optical fiber is single-mode fiber, and there are few transmission lines using the multimode fiber, there can be a case in which no MMF dedicated equipment is developed and equipment specifically developed for single-mode fiber transmission is used.
In the case of the MMF transmission, it is necessary to consider the band characteristics of a specific MMF fiber in addition to the transmission line loss. Additionally if SMF dedicated equipment is deployed, it is further necessary to consider coupling loss between the transmission line (MMF) and reception equipment (dedicated for SMF), and the effects of modal noise generated by the inter-modal interference of the transmission line fiber. Modal noise refers to the degradation of the received waveform due to impossible reception of all modes at the receiver because of inter-modal interference in the transmission line fiber.
Among these losses, the transmission line loss can be predicted in advance, and the band characteristics of the fiber can be checked by determining the viability of transmission based on the characteristics observed at the receiver end, but the modal noise largely depends on the transmission line environment and cannot be calculated. Furthermore, it greatly affects coupling loss.
FIGS. 1A and 1B are explanatory views of the effect of inter-modal interference.
In the ideal case in which there is no inter-modal interference (FIG. 1A), the light intensity distribution of the propagating beam in its cross section is a Gaussian distribution on the receiving end with the highest power corresponding to the center of the core, and the coupling loss can be calculated to some extent. However, when there is interference among propagation modes (FIG. 1B), non-uniform light intensity distributions are generated at the MMF section.
In this case, when an SMF receiver is connected to the receiving end, the coupling loss between MMF/SMF is large. In the connection unit, only a specific mode component that generates inter-modal interference is subject to a large coupling loss, and the received data cannot be restored to the original state even after the optical/electrical conversion by the reception unit, thereby greatly degrading the transmission characteristic.
When a transmission line is SMF, there is only one mode in the transmission line (SMF). Therefore, there is no inter-modal interference, and there is no difference in core diameter at the receiving connection unit. As a result, fundamentally no coupling loss occurs. In reality, a coupling loss of a maximum of 0.5 dB occurs, but it is a loss detected uniformly in the single mode propagated by the single mode fiber. Therefore, no specific component is attenuated, and no degradation of the transmission characteristics occurs.
The conventional technology of patent literature 1 discloses a technology for measuring the optical loss by measuring the optical power.
[Patent Literature 1] Japanese Patent Laid-open Publication No. Hei 11-271178
As described above, when SMF dedicated equipment is deployed for MMF transmission, degradation of the transmission characteristics occurs, and the degree largely depends on the transmission line length, the transmission line environment, etc., but there is currently no method of predicting the occurrence of errors.